The present invention relates generally to surgical spinal stabilization and more specifically to the instrumentation and technique for inserting a spinal implant within the intervertebral disc space between adjacent vertabra. More particularly, one embodiment of the present invention includes a protective guide sleeve used in conjunction with posterior spinal implant surgery for protecting neural structures and guiding associated surgical instrumentation.
Chronic back problems cause pain and disability for a large segment of the population. The number of spinal surgeries to correct causes of back pain have steadily increased over the past several years. Most often, back pain originates from damage or defects in the spinal disks between adjacent vertebrae. The disk can be herniated or can be suffering from a variety of degenerative conditions, so that in either case, the anatomical function of the spinal disk is disrupted. The most prevalent surgical treatment for these types of conditions has been to fuse the two adjacent vertebrae surrounding the affected disk. In most cases, the entire disk will be removed except for the annulus by way of a discectomy procedure. Since the damaged disk material has been removed, something must be positioned within the intradiscal space to prevent the collapse of the space which results in damage to the nerves extending along the spinal column. The intradiscal space is often filled with bone or a bone substitute in order to prevent disk space collapse and to further promote fusion of the two adjacent vertebrae.
There have been an extensive number of attempts made to develop an acceptable intradiscal implant that could be used to replace a damaged disk and maintain the stability of the disk space between adjacent vertebrae, at least until complete arthrodesis is achieved. These interbody fusion devices have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are represented by patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In these cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be pounded into the intradiscal space and the vertebral end plates. These types of devices are represented by the patents to Brantigan, U.S. Pat. Nos. 4,743,256, 4,834,757 and 5,192,327.
Various surgical methods have been devised for the implantation of fusion devices into the intradiscal space. Both anterior and posterior surgical approaches have been used for interbody fusions. In 1956, Ralph Cloward developed a method and instrumentation for anterior spinal interbody fusion of the cervical spine. Cloward surgically removed the disk material and placed a tubular drill guide with a large foot plate and prongs over an aligner rod and then embedded the prongs into adjacent vertebrae. The drill guide served to maintain the alignment of the vertebrae and facilitated the reaming out of bone material adjacent the disk space. The reaming process created a bore to accommodate a bone dowel implant. The drill guide was thereafter removed following the reaming process to allow for the passage of the bone dowel which had an outer diameter significantly larger than the reamed bore and the inner diameter of the drill guide. The removal of the drill guide left the dowel insertion phase completely unprotected. Thus, Cloward's method and instrumentation was designed for and limited to an anterior surgical approach and was inappropriate for a posterior application.
Furthermore, B. R. Wilterberger described in a paper entitled "Dowel Intervertebral Fusion as Used in Lumbar Disc Surgery" (published in The Journal of Bone and Joint Surgery, volume 39A, pgs. 234-92, 1957), the unprotected drilling of a hole from the posterior into the lumbar spine between the nerve roots and across the disk space, and then inserting a bone dowel into that disk space. While Wilterberger had taken the Cloward concept of circular drilling and dowel fusion and applied it to the lumbar spine from a posterior approach, he had not provided for an improved method, nor had he advanced the instrumentation so as to make that procedure significantly safe. Therefore, the Wilterberger procedure rapidly feel into disrepute.
Thereafter, a patent to Michelson, U.S. Pat. No. 5,484,437 disclosed a technique and associated instrumentation for inserting a fusion device from a posterior surgical approach. As described in more detail in the '437 patent, the surgical technique involves the use of a distractor having a penetrating portion that urges the vertebral bodies apart to facilitate the introduction of the necessary surgical instrumentation. The long distractor can act as a guide for drilling and reaming tools concentrically advanced over the outside of the distractor to prepare the site for the insertion of the fusion device. The '437 patent also discloses a hollow sleeve having teeth at one end that are driven into the vertebrae adjacent the disk space created by the distractor. These teeth maintain the disk space height during subsequent steps of the procedure following removal of the distractor. In accordance with one aspect of the Michelson invention, a drill is passed through the hollow sleeve to remove portions of the disk material and vertebral bone to produce a prepared bore for insertion of the fusion device. The drill is then removed from the sleeve and the fusion device is positioned within the disk space using an insertion tool.
While the Michelson technique and instrumentation represent a significant advance over prior surgical procedures for the preparation of the disk space and insertion of the fusion device, it has limitations. One such limitation is that the Michelson technique and instrumentation requires extensive removal of lamina and facet bony structures prior to commencement of the procedure to allow for the hollow sleeve to be positioned adjacent the fusion site. Because the facet joint generally adds stability to the spinal posterior column in a variety of loading modes, generally resisting sheer, torsion and flexion loads via its configuration in combination with the capsular ligaments, it would be beneficial to limit facet and lamina removal to the exact amount required for implant insertion While a greater amount of lateral facet removal generally requires less medial retraction of the dura to allow for the unimpeded insertion of the protective guide sleeve, this increased removal of bony structure decreases the overall stability of the spine. Alternately, if a smaller implant is used to thereby reduce the amount of vertebral bone removal, the result will be lesser intradiscal distraction and reduced tensioning of the annulus. Furthermore, a smaller surface area of engagement between the implant and the adjacent vertebrae will result in reduced stability and a decreased likelihood of interbody fusion over this reduced decorticated surface.
Thus, procedures and instruments that preserve the integrity of the surgical site, and more specifically, the spinal posterior column, are desirable. Although the prior techniques and instrumentation are steps in the right direction to accurately prepare a fusion site for insertion of a spinal implant, the need for improvement still remains. The present invention is directed to this need in the field and fulfills the need in a novel and unobvious way.